1. Field of the Invention
The present invention relates to a seal, and more specifically to a bellows gas seal capable of undergoing a reverse pressure change.
2. Discussion of the Related Art
Pumps, especially those in refineries and chemical plants, often handle difficult-to-seal liquids, including propane, butane, and other unstable, combustible, or toxic liquids. These liquids can cause short seal life and undesirable product leakage, prompting the passage of state health and safety pump emission regulations.
Dry-running gas lubricated seals have been developed and provide significant economic benefits over contact seals. One of the first working examples of a gas type seal was disclosed in U.S. Pat. No. 3,499,653 to Gardner. In that seal, the relatively rotatable sealing members are kept from touching one another during operation, reducing wear and heat due to friction. A gas pressure is created between the seal member faces to separate the faces by forming a thin film of gas between them. Shallow spiral grooves on the outer periphery of one of the seal members cause gas to be forced inward, thus providing sufficient pressure to separate the seal member faces. This type of seal is called a "non-contacting" seal because the sealing faces separate and a thin film of gas forms therebetween during operation. An O-ring is used as a secondary seal, and springs provide a biasing force against the opening of the seal faces and close the gap between the faces when the equipment is not operating.
More recently, gas seals have incorporated bellows as a secondary seal and biasing member. Often, a buffer gas is supplied to a seal at a pressure higher than the process fluid to help prevent leakage of process fluid across the non-contacting sealing faces. Should a sudden drop of buffer gas pressure occur so that the pressure of the process fluid exceeds that of the buffer gas (a so called "pressure reversal"), a non-contacting bellows gas seal may blow open, resulting in high leakage of process fluid into the atmosphere.